This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are internally water cooled so that metal shells form on the moving casting roll surfaces. The metal shells are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle or nozzles located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the casting rolls above the nip and extending the length of the nip. The casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls to restrict the casting pool against outflow. The upper surface of the casting pool (generally referred to as the “meniscus” level) is usually above the lower end of the delivery nozzle so that the lower end of the delivery nozzle is immersed within the casting pool.
During casting, the casting rolls rotate such that the metal from the casting pool solidifies into metal shells on the casting rolls that are brought together at the nip to produce a cast strip downwardly from the nip. One of the difficulties in the past during the casting operation has been chatter. Chatter is a phenomenon where the casting machine vibrates typically around one of two main frequencies, generally about 35 hertz (Hz) and 65 hertz (Hz).
It has been proposed that chatter is generated when the metal shells solidified on the moving surfaces of the casting rolls are brought together at the nip and rub and interact against each other. The metal shells have many small raised areas. A wide spectrum of frequencies is generated when these small raised areas rub and interact with each other. These excite the natural frequencies of the casting machine system during the casting operation and the casting machine vibrates at these natural frequencies creating chatter.
In addition, vibration of the casting rolls at the natural frequencies also causes disturbances at the meniscus. These disturbances cause variation in the solidification process, which in turn, when they reach the nip, reinforces the vibration of the casting rolls. Hence, the chatter is further amplified and modulated by this regenerative mechanism.
Chatter should be avoided because of the surface defects and thickness variation chatter causes in the cast strip. When chatter becomes severe, horizontal lines may be observed across the width of the cast strip. If chatter is extreme, breakage of the strip may occur.
It has been previously suggested to reduce chatter by lowering the casting roll separation force and allowing “mushy” material (i.e. liquid metal between the metal shells) to be “swallowed” between the metal shells during casting. However, the problem with this approach was that if the casting roll force is lowered too much so the gap between the casting rolls is too large, the mushy material between the metal shells will cause defects in the strip such as ridges. To further explain, immediately below the nip, the mushy material in the strip is in communication with the casting pool due to the ferrostatic pressure. The mushy material releases energy to the cast strip just after exiting the nip. As a result, the surface of the strip gets too hot and yields under the influence of the ferrostatic head from the casting pool, causing surfaces defects known as ridges in the cast strip. Therefore, there is still a need for an efficient method to reduce chatter during the casting operation.
We have found a method to reduce chatter by a controlled oscillation of the gap between the casting rolls allowing a controlled intermittent amount of mushy material between the metal shells providing dampening of the system and reducing chatter during a casting operation. The mushy material may include molten metal and partial solidified metal, and includes all the material between the metal shells not sufficiently solidified to be self-supporting.
It has also been found that chatter may be reduced by oscillating the casting speed. The casting speed may be oscillated at an amplitude between ±1 and ±4 m/min and at a frequency between 1 and 5 hertz. Further, the casting speed may be oscillated at an amplitude between ±2 and ±3 m/min and at a frequency between 2 and 4 hertz.
Currently disclosed is a method of casting thin strip comprising the steps of: assembling a pair of counter-rotating casting rolls laterally forming a gap between circumferential casting surfaces of the casting rolls at a nip between the casting rolls through which metal strip can be cast; assembling side dams adjacent end portions of the casting rolls to permit a casting pool of molten metal to be formed and supported by the casting surfaces of the casting rolls; assembling a metal delivery system above the casting rolls adapted to deliver molten metal to form the casting pool supported on the casting surfaces of the casting rolls above the gap and confined by the side dams; counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel inwardly toward the nip to form metal shells on the surfaces of the casting rolls and deliver a cast strip downwardly from the gap between the casting rolls with a mushy internal portion; and providing a drive mechanism to oscillate the gap between the casting rolls at an amplitude between ±5 and ±50 microns (or μm) at a frequency between 1 and 7 hertz (or Hz) to vary thickness of the mushy internal portion in the cast strip and reduce chatter during casting.
The oscillating of the gap between the casting rolls at the nip may be performed by sinusoid oscillation. Alternatively, the oscillating of the gap between the casting rolls at the nip may be provided by a periodic function, for example a step function, to change the gap between the casting rolls.
The gap between the casting rolls at the nip may be oscillated at an amplitude between ±10 and ±40 microns (or μm). Furthermore, the gap between the casting rolls at the nip may be oscillated at an amplitude between ±20 and ±30 microns (or μm). Additionally, the gap between the casting rolls at the nip may be oscillated between at a frequency between 2 and 5 hertz (or Hz).
Also disclosed is an apparatus for casting thin strip. The apparatus comprising at least a pair of counter-rotating casting rolls where each casting roll having a circumferential casting surface and a pair of end portions. A lateral gap is formed between the circumferential casting surfaces of each casting roll at a nip between the casting rolls through which a metal strip can be cast. At least a pair of side dams are adjacent the end portions of the casting rolls to permit a casting pool of molten metal to be formed supported by the casting surfaces of the casting rolls. A metal delivery system above the casting rolls is provided for delivering molten metal to form the casting pool supported by the casting surfaces of the casting rolls above the lateral gap and confined by the side dams. Finally, a drive mechanism is provided to oscillate the gap between the casting rolls at an amplitude between ±5 and ±50 μm at a frequency between 1 and 7 hertz to vary a thickness of a mushy material in the cast strip and reduce chatter during casting.